The invention relates generally to medical imaging. In particular, the invention relates to digital X-ray medical imaging systems having a flat-panel digital X-ray detector.
The bone mineral density (BMD) of a bone reflects the strength of the bone as represented by calcium content. It is defined as the integral mass of bone mineral per unit of projected area in grams per square centimeter. BMD is a useful tool for the diagnosis and treatment of several diseases and conditions, one of which is osteoporosis.
Osteoporosis is a disease of bone in which the BMD is reduced due to depletion of calcium and bone protein. Osteoporosis predisposes a person to fractures, which are often slow to heal and heal poorly. It is more common in older adults, particularly post-menopausal women; in patients on steroids; and in those who take steroidal drugs. Unchecked osteoporosis can lead to changes in posture, physical abnormality (particularly a condition known colloquially as “dowager's hump”), and decreased mobility. Treatment of osteoporosis includes ensuring that the patient's diet contains adequate calcium and other minerals needed to promote new bone growth, and for post-menopausal women, estrogen or combination hormone supplements.
Dual-energy X-ray absorptiometry (DXA or DEXA) is an increasingly important bone density measurement technology. In fact, osteoporosis is defined by the World Health Organization (WHO) as a BMD having a value 2.5 standard deviations below peak bone mass (in a 20-year-old sex-matched healthy person average) as measured by DXA. The fundamental principle behind DXA is the measurement of the transmission of X-rays with two different energy levels. By measuring how much X-ray energy is transmitted through the patient, the amount of X-ray energy that is absorbed in the patient can be determined. Soft tissues and bone absorb the two energy level X-rays to different degrees. As a result, the absorption of X-rays by the soft tissue may be distinguished from the absorption of X-rays by bone. The BMD is then determined from the bone image data.
Because of the limitations in the size of early digital X-ray detectors, techniques were developed to perform DXA by moving the detector in conjunction with an X-ray source to cover the desired region of interest, such as the hip or vertebra. Examples of this type of DXA are pencil beam DXA and fan beam DXA. In pencil beam DXA, the X-ray source and detector perform a two-dimensional raster scan of the region of interest. In fan beam DXA, a slit collimator is used to produce a fan-shaped beam that extends across the region of interest such that the region of interest may be covered with a single sweep of the source and detector.
When X-rays interact with tissues and bone in a patient, some of the X-rays are deflected or redirected. These redirected X-rays are known as scatter. Scatter can produce a significant level of error in the quantitative values obtained during DXA. Other than having a different direction or energy, scattered X-rays are indistinguishable from the primary X-rays coming directly from the source and are included in the image used to derive the BMD. Thus, the intensity of X-rays that would appear to have been transmitted through the soft tissue and bone may be greater than the actual amount of radiation received at pixel locations of a detector due to the detection of scattered radiation. Conversely, the intensity of X-rays that would appear to have been transmitted through the soft tissue and bone would appear to be less than the actual amount that would have received the X-rays if not for scatter. In addition to affecting the clarity of the image, the values for the BMD would also be affected by scatter.
Existing DXA systems have narrow collimation and a small field of view which enables these systems to minimize the effects of scatter. However, digital X-ray detector technology has now advanced to the point where large flat-panel digital X-ray detectors are large enough to cover significantly-sized regions of clinical interest. As a result, the digital X-ray detector may remain stationary relative to the patient. In addition, the effect of scatter is much greater than in existing DXA systems. Techniques to reduce scatter during acquisition, such as anti-scatter grids and air gaps, may be used to reduce scatter. However, these techniques attenuate the X-rays and do not completely remove the effect of scatter. As a result, the patient must be exposed to greater amounts of X-rays to achieve the desired results.
Therefore, a technique is desired that would reduce the effects of scatter when a large, flat-panel digital detector is used. In particular, a technique is desired that would reduce the effects of scatter, while reducing, or at least not increasing, a patient's exposure to X-rays.